How to access global variable when there is a local and global conflict

Question

Code :

int a = 33;
int main()
{
  int a = 40; // local variables always win when there is a conflict between local and global.
  // Here how can i access global variable 'a' having value '33'.
}

If you ask : Why would someone want to do above thing? Why [a-zA-Z]* ?

My answer would be : Just to know that 'it is possible to do so'.

Thanks.

http://stackoverflow.com/questions/5827447/how-refer-to-a-local-variable-share-same-name-of-a-global-variable-in-c — Ahmed Aeon Axan, Mar 10 '13 at 20:04
It means any question starting with 'Why' and ending on '?' e.g. Why you want to access global here? or why blah blah blah blah ? — VishalDevgire, Mar 10 '13 at 20:16

score 15 · Accepted Answer · answered Mar 10 '13 at 20:06

15

How about this old trick:

int main()
{
    int a = 40; // local variables always win when there is a conflict between local and global.

    {
        extern int a;
        printf("%d\n", a);
    }
}

answered Mar 10 '13 at 20:06

cnicutar

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3

+1 for beating me by 30 seconds. – R.. GitHub STOP HELPING ICE Mar 10 '13 at 20:07
4

@R.. To be fair, I think I first saw it some time ago in one of your older posts. – cnicutar Mar 10 '13 at 20:08
@cnicutar when would you prefer to access via pointer compare to extern keyword **?** I think it will work for global static variable too. correct **?** – Grijesh Chauhan Mar 10 '13 at 20:24
1

@GrijeshChauhan I think it's best to avoid name collisions in the first place, by defensively naming objects and functions your code exports (i.e. everything not `static`). On the other hand, if you already have a pointer there's no need for this trick. – cnicutar Mar 10 '13 at 20:25
it didn't work, I'm using DigitalMars compirer – void Jun 23 '19 at 17:50

R.. GitHub STOP HELPING ICE · Answer 2 · 2013-03-10T23:29:01.207

11

int a = 33;
int main()
{
  int a = 40;
  int b;
  {
    extern int a;
    b = a;
  }
  /* now b contains the value of the global a */
}

A harder problem is getting a if it's static with file scope, but that's also solvable:

static int a = 33;
static int *get_a() { return &a; }
int main()
{
  int a = 40;
  int b = *get_a();
  /* now b contains the value of the global a */
}

edited Mar 10 '13 at 23:29

answered Mar 10 '13 at 20:07

R.. GitHub STOP HELPING ICE

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Why not just do `int b = a; int a = 40;`? Or, might the compiler reorder the lines in this case.... – ad absurdum Sep 18 '17 at 09:58
Indeed you could do that. – R.. GitHub STOP HELPING ICE Sep 18 '17 at 12:58

masoud · Answer 3 · 2013-03-10T21:35:10.470

4

IT IS C++, I OVERLOOKED C tag, SORRY !

int a = 100;

int main()
{
    int a = 20;

    int x = a; // Local, x is 20

    int y = ::a; // Global, y is 100

    return 0;
}

edited Mar 10 '13 at 21:35

answered Mar 10 '13 at 20:41

masoud

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How to access global variable when there is a local and global conflict

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